-- Copyright 2023 Lennart Augustsson
-- See LICENSE file for full license.
module Data.Either(module Data.Either) where
import Prelude()              -- do not import Prelude
import Primitives
import Control.Applicative
import Control.Monad
import Data.Bool
import Data.Eq
import Data.Function
import Data.Functor
import Data.Int
import Data.Ord
import Text.Show

data Either a b = Left a | Right b
  deriving (Eq, Ord)

either :: forall a b r . (a -> r) -> (b -> r) -> Either a b -> r
either f _ (Left  a) = f a
either _ f (Right b) = f b

isLeft :: forall a b . Either a b -> Bool
isLeft (Left  _) = True
isLeft (Right _) = False

isRight :: forall a b . Either a b -> Bool
isRight (Left  _) = False
isRight (Right _) = True

instance forall a b . (Show a, Show b) => Show (Either a b) where
  showsPrec p (Left  a) = showParen (p>=appPrec1) (showString "Left "  . showsPrec appPrec1 a)
  showsPrec p (Right b) = showParen (p>=appPrec1) (showString "Right " . showsPrec appPrec1 b)

instance forall a . Functor (Either a) where
  fmap _ (Left a) = Left a
  fmap f (Right b) = Right (f b)

instance forall a . Applicative (Either a) where
  pure b              = Right b
  Right f <*> Right x = Right (f x)
  Right _ <*> Left a  = Left a
  Left a  <*> _       = Left a

instance forall a . Monad (Either a) where
  Right b >>= k = k b
  Left a  >>= _ = Left a